Slow Heartbeat: What Could Be Causing It

What Bradycardia Actually Means

Bradycardia is defined as a heart rate below 60 beats per minute at rest. The threshold is not absolute. A well-trained endurance athlete with a resting rate of 45 bpm and zero symptoms needs no treatment, while a sedentary 72-year-old at 55 bpm with recurrent near-syncope may need urgent evaluation.

The heart's electrical impulse originates in the sinoatrial (SA) node, a cluster of specialized cells in the right atrium that fires 60 to 100 times per minute under normal conditions. From the SA node, the signal travels through the atrioventricular (AV) node, down the bundle of His, and into the Purkinje fibers to trigger ventricular contraction [1]. Bradycardia results when this chain slows or breaks at any point.

The 2018 ACC/AHA/HRS Guideline for the Evaluation and Management of Patients With Bradycardia classifies the condition by the site of dysfunction: sinus node disease, AV conduction disorders, or extrinsic causes such as drugs and metabolic abnormalities [2]. That classification matters because it dictates whether the fix is stopping a medication, replacing thyroid hormone, or implanting a pacemaker.

Prevalence data vary by population. A community-based study published in the European Heart Journal found sinus bradycardia in 3.3% of men and 1.8% of women over 65, with incidence rising sharply after age 75 [3]. In younger adults, the rate is far lower unless athletic conditioning or medications are involved.

Medications: The Most Common Reversible Cause

Beta-blockers, non-dihydropyridine calcium channel blockers (verapamil, diltiazem), digoxin, amiodarone, and clonidine are the drugs most frequently responsible for clinically significant bradycardia. If a patient on metoprolol 100 mg twice daily presents with a heart rate of 44 bpm and lightheadedness, the medication is the leading suspect.

The 2018 ACC/AHA/HRS bradycardia guideline gives a Class I recommendation to identify and discontinue or dose-reduce offending agents before considering permanent pacing [2]. This step alone resolves the problem in a large proportion of cases. A retrospective analysis of 277 patients referred for pacemaker evaluation at a single academic center found that 16% had drug-induced bradycardia that resolved after medication adjustment [4].

Digoxin deserves special mention. Its therapeutic window is narrow (serum level 0.5 to 0.9 ng/mL for heart failure, per the DIG trial post hoc analysis), and toxicity causes both bradyarrhythmias and tachyarrhythmias [5]. Renal impairment, hypokalemia, and drug interactions with amiodarone or verapamil raise digoxin levels unpredictably. Any patient on digoxin with new bradycardia should have a stat serum digoxin level drawn.

Polypharmacy amplifies risk. Combining a beta-blocker with diltiazem, a pattern sometimes seen when rate control for atrial fibrillation is aggressive, can produce profound bradycardia. The FDA has issued safety communications about the risk of serious bradycardia when amiodarone is co-administered with sofosbuvir-containing hepatitis C regimens [6].

Sinus Node Dysfunction (Sick Sinus Syndrome)

Sinus node dysfunction (SND) is a degenerative condition in which the SA node fails to generate impulses at an adequate rate. It is the single most common indication for permanent pacemaker implantation, accounting for roughly half of all new pacemaker implants in the United States each year [7].

SND encompasses several ECG patterns: persistent sinus bradycardia, sinus pauses or arrest, sinoatrial exit block, and tachy-brady syndrome, where episodes of atrial tachycardia or fibrillation alternate with periods of marked bradycardia. Tachy-brady syndrome is particularly difficult to manage because drugs used to control the tachycardia (beta-blockers, calcium channel blockers, antiarrhythmics) worsen the bradycardia.

The pathophysiology involves fibrosis and fatty infiltration of the SA node, a process that accelerates with age. Histological studies have shown a progressive loss of pacemaker cells beginning in the fifth decade, with up to 90% cell loss by age 75 in some specimens [8]. Ischemic heart disease, infiltrative diseases like amyloidosis and sarcoidosis, and prior cardiac surgery (especially atrial procedures) can also damage the node.

Symptoms range from fatigue and exercise intolerance to frank syncope. The key diagnostic challenge is correlating symptoms with documented bradycardia. A 24-hour Holter monitor captures intermittent events, but if episodes are infrequent (less than once per week), a 14-day event monitor or an implantable loop recorder may be needed [2].

AV Conduction Disorders

Atrioventricular block occurs when impulse transmission from the atria to the ventricles is delayed or completely interrupted. There are three degrees, and the clinical significance varies dramatically between them.

First-degree AV block (PR interval greater than 200 ms) is common, present in about 1.6% of young adults and up to 6% of those over 60 [9]. It is almost always benign. No treatment is required unless the PR interval is extremely prolonged (greater than 300 ms) and causing symptoms due to loss of AV synchrony.

Second-degree AV block comes in two types. Mobitz type I (Wenckebach) shows progressive PR prolongation before a dropped beat; it is usually located in the AV node, responds to atropine, and rarely requires pacing. Mobitz type II, by contrast, shows sudden dropped beats without prior PR prolongation, localizes to the His-Purkinje system, and carries a high risk of progression to complete heart block. The 2018 ACC/AHA/HRS guideline gives a Class I recommendation for permanent pacing in symptomatic Mobitz type II block [2].

Third-degree (complete) AV block means no atrial impulses reach the ventricles. The ventricles are sustained by an escape rhythm, which may be junctional (40 to 60 bpm, narrow QRS, relatively stable) or ventricular (20 to 40 bpm, wide QRS, unreliable). Complete heart block with a ventricular escape rhythm is an emergency. Temporary pacing should be initiated immediately, followed by permanent pacemaker implantation.

Causes of AV block include idiopathic fibrosis of the conduction system (Lenegre disease), ischemic heart disease (inferior MI often produces transient AV block; anterior MI produces AV block with worse prognosis), Lyme disease, endocarditis, and medications [10].

Hypothyroidism and Other Metabolic Causes

Thyroid hormone has direct chronotropic and dromotropic effects on the heart. Hypothyroidism decreases SA node firing rate, slows AV conduction, and reduces cardiac output. The association is well established: a study in the Archives of Internal Medicine found that even subclinical hypothyroidism (TSH 4.5 to 10 mIU/L) was associated with a resting heart rate 2 to 4 bpm lower than euthyroid controls [11].

Overt hypothyroidism (TSH above 10 mIU/L) produces more pronounced bradycardia. In severe myxedema, heart rates in the 30s and 40s are not unusual. The treatment is thyroid hormone replacement with levothyroxine, typically starting at 1.6 mcg/kg/day in younger patients without cardiac disease. In older patients or those with known coronary artery disease, the starting dose should be low (25 to 50 mcg/day) and titrated slowly to avoid precipitating angina or arrhythmia [12].

Other metabolic causes include hyperkalemia (which slows conduction and can progress to sine wave pattern and cardiac arrest at levels above 7.0 mEq/L), severe hypoxia, hypothermia (the Osborn or J wave appears on ECG when core temperature falls below 32°C), and elevated intracranial pressure (Cushing reflex: hypertension with reflex bradycardia) [13].

Obstructive sleep apnea is an underrecognized contributor. During apneic episodes, vagal surges can produce sinus pauses exceeding 3 seconds. A study in the American Journal of Respiratory and Critical Care Medicine documented nocturnal bradycardia in 18% of patients with moderate-to-severe OSA [14]. Treatment of the apnea with CPAP often resolves the bradycardia, avoiding the need for a pacemaker.

Athletic Bradycardia: When Slow Is Normal

Endurance athletes commonly have resting heart rates between 40 and 50 bpm, and elite athletes sometimes register rates in the low 30s during sleep. This is a physiologic adaptation driven by increased vagal tone, cardiac remodeling with greater stroke volume, and intrinsic changes in the SA node [15].

The clinical question is whether bradycardia in an athlete is benign adaptation or a sign of pathology. The 2020 European Society of Cardiology guidelines on sports cardiology state that resting bradycardia in an athlete is concerning only if it is accompanied by symptoms (syncope, presyncope, exercise intolerance) or associated with prolonged sinus pauses (>3 seconds while awake) [16].

First-degree AV block and Mobitz type I (Wenckebach) second-degree AV block are also common in athletes and almost always resolve with exercise. If a 25-year-old marathon runner with a resting rate of 38 bpm can increase heart rate appropriately during a treadmill stress test and has no symptoms, reassurance is the only intervention needed.

Detraining can help differentiate the two scenarios. If heart rate normalizes after 3 to 6 months of reduced training, the bradycardia was physiologic. Persistence of marked bradycardia or high-degree AV block after detraining suggests intrinsic conduction disease.

The Diagnostic Workup

A systematic approach prevents both missed diagnoses and unnecessary pacemakers. The 2018 ACC/AHA/HRS guideline recommends the following sequence [2].

Step 1: History and medication review. Catalog every drug, including eye drops (timolol, a beta-blocker, causes systemic bradycardia) and supplements. Ask about exercise habits, sleep quality (screening for OSA), and symptoms of hypothyroidism.

Step 2: 12-lead ECG. This identifies the rhythm, heart rate, PR interval, QRS duration, and any signs of ischemia or structural disease. It is the single most informative test.

Step 3: Basic labs. TSH, electrolytes (potassium, calcium, magnesium), digoxin level if applicable, and troponin if ischemia is suspected.

Step 4: Ambulatory monitoring. If the ECG does not capture the arrhythmia, use a Holter monitor (24 to 48 hours) for daily symptoms, a patch monitor (7 to 14 days) for weekly symptoms, or an implantable loop recorder (up to 3 years) for rare events.

Step 5: Electrophysiology study. Reserved for cases where noninvasive testing is inconclusive and pacing is being considered. The study measures sinus node recovery time and His-ventricular (HV) interval to localize conduction disease.

Step 6: Cardiac imaging. Echocardiography to assess ventricular function and wall thickness. Cardiac MRI if infiltrative disease (sarcoidosis, amyloidosis) is suspected, particularly when AV block occurs in a young patient without an obvious cause [17].

Treatment: From Medication Adjustment to Pacemaker

Treatment follows a clear hierarchy. Reverse any identified cause first. Stop or reduce the offending drug. Replace thyroid hormone. Treat the sleep apnea. Correct the hyperkalemia.

For acute symptomatic bradycardia (hemodynamic instability, altered mental status, chest pain), the AHA's 2020 ACLS algorithm recommends atropine 0.5 mg IV, repeated every 3 to 5 minutes to a maximum dose of 3 mg [18]. Atropine works by blocking vagal input to the SA and AV nodes. It is ineffective in infranodal block (Mobitz type II, complete heart block with wide QRS) because the block is below the level of vagal innervation. In those cases, proceed directly to transcutaneous pacing or a transvenous temporary pacing wire.

Isoproterenol (2 to 10 mcg/min IV infusion) and dopamine (5 to 20 mcg/kg/min) are second-line agents for bradycardia unresponsive to atropine while pacing is being arranged [18].

Permanent pacemaker implantation is indicated when bradycardia is symptomatic and no reversible cause exists. The 2018 guideline provides Class I indications including: symptomatic sinus node dysfunction, symptomatic or advanced second-degree AV block, third-degree AV block, alternating bundle branch block, and bradycardia with pauses >3 seconds producing symptoms [2].

Modern pacemakers are dual-chamber (DDD) devices in most cases, preserving AV synchrony. Single-chamber ventricular (VVI) pacing is reserved primarily for patients with permanent atrial fibrillation and slow ventricular response. Leadless pacemakers (Micra, Aveir) are an option for selected patients, particularly those with limited venous access or high infection risk [19].

According to the American College of Cardiology's National Cardiovascular Data Registry, in-hospital complication rates for new pacemaker implantation are approximately 3.4%, with lead dislodgement (1.4%) and pneumothorax (0.7%) being the most common [20]. Mortality at 30 days is under 0.5% for elective implants.

Less Common but Serious Causes

Certain diagnoses carry high stakes and should not be missed.

Lyme disease. Borrelia burgdorferi has a tropism for cardiac conduction tissue. Lyme carditis causes AV block (often high-degree or complete) in about 1% of untreated Lyme infections. The block is usually reversible with IV ceftriaxone 2 g daily for 14 to 21 days, and permanent pacing can often be avoided if the diagnosis is made promptly [21]. The CDC recommends considering Lyme carditis in any patient under 50 with new AV block and possible tick exposure.

Cardiac sarcoidosis. Noncaseating granulomas infiltrate the myocardium, with a predilection for the basal interventricular septum where the conduction system runs. Cardiac sarcoidosis should be suspected when AV block occurs in a patient aged 30 to 55, particularly with a history of bilateral hilar lymphadenopathy. Cardiac MRI showing late gadolinium enhancement in a non-coronary distribution supports the diagnosis [17]. These patients often need both a pacemaker and immunosuppressive therapy.

Infiltrative cardiomyopathy (amyloidosis). Both AL and ATTR amyloidosis cause conduction disease. ATTR amyloidosis, once considered rare, is increasingly recognized with the availability of technetium pyrophosphate (Tc-PYP) nuclear scanning. A study in JAMA Cardiology found ATTR cardiac amyloidosis in 13.3% of patients aged 60 and older hospitalized with heart failure with preserved ejection fraction [22]. Tafamidis 80 mg daily reduces mortality and cardiovascular hospitalizations in ATTR cardiomyopathy, per the ATTR-ACT trial (N=441) [23].

Inherited channelopathies. Mutations in SCN5A and other ion channel genes can cause progressive conduction disease in younger patients, sometimes presenting as isolated first-degree AV block that worsens over decades. A family history of pacemaker implantation before age 50 should prompt genetic evaluation.

When to Seek Immediate Medical Attention

Not all slow heart rates are emergencies, but some are. Go to an emergency department if bradycardia occurs with syncope or near-syncope, chest pain, severe shortness of breath, confusion, or a heart rate below 40 bpm with any symptoms. Acute complete heart block and symptomatic Mobitz type II block require inpatient monitoring and likely temporary pacing within hours.

For patients with asymptomatic bradycardia discovered on a routine exam or wearable device, a non-urgent outpatient evaluation within 1 to 2 weeks is appropriate. The initial workup (ECG, TSH, medication review) can be completed at a primary care visit, with cardiology referral if the ECG shows conduction disease beyond simple sinus bradycardia.

Wearable devices (Apple Watch, Fitbit, Garmin) have increased the detection of asymptomatic bradycardia. The Apple Heart Study (N=419,297) primarily examined atrial fibrillation detection, but its infrastructure demonstrated that consumer wearables can generate clinically actionable heart rate data [24]. A slow rate detected on a wearable in an asymptomatic individual warrants confirmation with a medical-grade ECG before initiating a workup.